This invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle when two-wheel drive is adequate and to provide torque for both drive axles automatically when four-wheel drive is required. The torque transfer case incorporates a double-acting overrunning clutch for automatically engaging and disengaging the four-wheel drive function.
In recent years there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which direct torque to one output to drive a fixed axle and through an overrunning clutch to another output for driving a steering axle. The clutch automatically engages and disengages the four-wheel drive function. However, an excessive overrunning condition may cause undesirable clutch lockup, thus negating its automatic function. One attempt at a solution to this problem was the use of a speed-sensitive blocking structure, as disclosed in U.S. application Ser. No. 615,001 filed Sept. 19, 1975. It was then found that an excessive overrunning condition may obtain at speeds below the threshold speed of such a structure. Thus, there was still a need to provide an overrunning clutch which engages and disengages automatically, but which prevents undesirable engagement even at low speeds. This need now is believed to have been met by a blocking structure which is direction-sensitive rather than speed-sensitive. Such a direction-sensitive blocking structure is disclosed in U.S. application Ser. No. 691,672 filed June 1, 1976, which application is a continuation-in-part of U.S. application Ser. No. 637,923 filed Dec. 5, 1975.
Subsequently, it was found that under certain operating conditions there is a tendency for the fixed axle wheels to skid, with consequent loss of vehicle control. As disclosed in U.S. application Ser. No. 676,774 filed Apr. 14, 1976, this tendency may be corrected by providing sufficient drive back from the steering axle wheels to keep the fixed axle wheels rotating. Drive may be established between the axles independently of the clutch by loosely connecting one clutch race element with the steering axle, and by providing frictional engagement between this element and the fixed axle.
As a result of the action of such friction drive, undesirable clutch lockup in the wrong engaged position can occur when, for example, the vehicle is brought to a stop in a turning configuration and more than a single direction reversal is initiated. This can occur notwithstanding incorporation in the clutch of blocking devices known heretofore.